As methods for smelting nickel oxide ore which may also be called limonite or saprolite, known are a dry smelting method for producing nickel matte using a flash smelting furnace, a dry smelting method for producing an iron-nickel alloy (ferronickel) using a rotary kiln or moving hearth furnace, a wet smelting method for producing mixed sulfide using an autoclave and the like.
Dry smelting of nickel oxide ore commonly includes roasting the ore in a rotary kiln, and then melting the roasted ore in an electric furnace to obtain a ferronickel metal, and then separating slag. At this time, some iron is allowed to remain in the slag for maintaining the concentration of nickel in the ferronickel metal at a high level. However, it disadvantageously requires a large amount of electric energy because the whole amount of nickel oxide ore needs to be melted to generate slag and a ferronickel.
Patent Document 1 discloses a method including inputting nickel oxide ore and a reducing agent (anthracite) into a rotary kiln, and reducing the ore in a semi-molten state to reduce parts of nickel and iron into metal, and then recovering a ferronickel by gravity separation or magnetic separation. Advantageously, according to the above method, a ferronickel metal can be obtained without performing electric melting, leading to reduced energy consumption. However, the method suffers from the following problems: reduction is performed in a semi-molten state, and thus the produced metal will be dispersed in the form of small particles; and the yield of nickel metal will be relatively low partly due to losses during gravity separation and magnetic separation.
Further, Patent Document 2 discloses a method for producing a ferronickel using a moving hearth furnace. The method described in the above document includes mixing raw materials containing nickel oxide and iron oxide with a carbonaceous reducing agent to form a pellet, and heat-reducing the mixture in a moving hearth furnace to obtain a reduced mixture, and then melting the reduced mixture in a separate furnace to obtain a ferronickel. The document describes that alternatively, both slag and metal or one of either may be melted in a moving hearth furnace. However, melting the reduced mixture in a separate furnace requires a large amount of energy as in the melting process in an electric furnace. Further, disadvantageously, the slag and the metal may be fused to the furnace floor when melted in the furnace, resulting in difficult discharge from the furnace.
Furthermore, almost all of ferronickel recovered from nickel oxide ore, including limonite, saprolite, and the like, will serve as a raw material of stainless steel. Ferronickel with a high concentration of nickel is preferred for a raw material of stainless steel. Ferronickel with a nickel grade of 4% or more is usually sold at a price in accordance with the international standard price of LME. Disadvantageously, ferronickel with a nickel grade of less than 4%, on the other hand, may not be easily sold.
Patent Document 1: Japanese Examined Patent Application Publication No. H01-21855
Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2004-156140